BTW this could have some spoilers.
I'm farther ahead in the story so I dont remembered what ch's 1-24 covered already and what they didn't. But I think this explains what you're confused about.
Again sorry if their are spoilers in this, im not sure.
Honestly this part was confusing to me too back when I first read it.
What I think its trying to say is that back when Dohyun and Sungho where having sex in the storage room in high school, Sungho was feeling terrible about it. He felt like Dohyun was, in a way, manipulating and using him to do what he wanted and was ignoring his feelings entirely. The diary entry's show this.
The blank pages were every time they had sex in high school, totaling up to 19 pages meaning they did it 19 times. The red marks on the calendar marked each time they did it.
The end page said he that he was sick of everything which I think meant Sungho stoped interacting with Dohyun (he might've ended the "relationship" they had, I'm not sure).
In the present Dohyun said that they had sex 8 times already, leaving 11 times left.
The reason for Sungho kidnapping and locking up Dohyun was because he wanted 'revenge' or just wanted Dohyun to know what he felt back when they were kids. He felt like Dohyun was playing with his feelings and Sungho wanted him to know how he felt back then. So he did this by recreating the warehouse they used to go to back in high school and recreating the exact scenarios when they had sex. He wanted to do it 19 times because he felt like if he did it the exact number of times they did it in high school, a part if him would somehow be satisfied and the emotions inside him would go away.
But as the story goes along well see if that works out for him or not .
Thank you so much loved your explanation!! Thank you so much for helping me!! Have a nice day thank you
I'm on chapter 24 but I'm enable to understand what is going with that dates and the calendar part pls help me!! To understand